Hot air impingement is one method for heating an object. This method is based on the transfer of heat from a fluid (e.g., a gas, such as air) that has a higher temperature to an object that has a lower temperature, thereby changing the internal energy of the fluid and the object in accordance with the first law of thermodynamics.
The fundamental principle of a heating apparatus is conversion of available power (e.g., electric power) into heat energy that is directed to, and absorbed by, an object located in the apparatus to raise the temperature of the object. Accordingly, achieving an optimal heating efficiency requires maximizing (1) the amount of heat energy that is converted from a given input power; (2) the amount of the heat energy that is directed to an object in the apparatus; and (3) the amount of the heat energy that is absorbed and retained by the object.
As an object resides in an apparatus that provides a surrounding hot air environment (such as an oven), temperature gradients, or several boundary layers, form around the cooler object. The apparatus heats the object by transferring heat energy to the object through these temperature gradients. Forced air convection by, for example, a fan can improve the heat transfer by “wiping away” the temperature gradients around the object and bringing the higher temperature air closer to the object.
Hot air impingement can further improve the heat transfer by “piercing” the temperature gradients with jets of hot air and bringing the air at higher temperature closer to the surface of the object. However, significant portions of the electric power and the heat energy from the hot air impingement are lost in the process to the walls of the apparatus, to various openings, and to the plenum and air blower that form the hot air circulation and delivery system of the apparatus.
As illustrated in FIG. 10A, hot air impingement uses columns 1604 of hot air from hot air jet holes 1606 that are disposed in an upper wall of a chamber (not shown), above an object 1602. One of ordinary skill in the art would understand that the hot air jet holes 1606 could also be disposed in a bottom surface of the chamber, or in a side wall of the chamber, so that the hot air impingement could occur from, respectively, below the object or a side of the object, as well. The columns 1604 are formed by moving the hot air through the holes 1606 at a high velocity. The chamber typically includes a return opening 1608 that is provided in its back wall.
As shown in FIG. 10A, only 19 of the 27 columns 1604 of air impact object 1602. This reduces the efficiency of the hot air impingement. As shown in FIG. 10B, another well-known problem with the technique of hot air impingement is “spotting” in the areas directly impacted by the hot air jets, causing uneven heating or scorching of the surface of the object 1602. While this problem may be resolved by, for example, reducing the hot air velocity and/or increasing the diameter of the columns 1604 of impinging hot air, such solutions may further reduce the efficiency of the hot air impingement. In addition, the diameter/cross-sectional area of a column of hot air impingement generally increases as the distance from the hot air jet orifice increases, thereby reducing the efficiency of hot air impingement. While this problem may be solved by increasing the hot air velocity, such solution may further aggravate the spotting problem.
Hot air jet holes 1606 are separated from each other by a distance that is sufficient to allow the air that rebounds off object 1602 to form various paths to return opening 1608. Such paths are shown in FIG. 10B. Referring to FIGS. 10A and 10B, after columns 1604 of hot air strike object 1602, the rebounded air from each column 1604 travels a different path to return opening 1608. In particular, the air does not follow a uniform path from the time it enters the chamber until it reaches the return opening. Because the path of the rebounded air to the return opening 1608 is somewhat circuitous, a large pressure drop is generated along the path. Consequently, more force is required to return the air to the blower that provides the air to the chamber (not shown).
In addition to traveling different paths, the rebounded air from each column 1604 of hot air travels, for the most part, different distances to return opening 1608 after striking object 1602. As seen in FIG. 10B, air that strikes object 1602 farthest away from return opening 1608 must travel the entire length of object 1608 to get to return opening 1608, while air that strikes object 1602 nearer return opening 1608 travels a shorter distance to reach return opening 1608. Accordingly, the portion of object 1602 near the return opening 1608 comes in contact with a larger volume of return air than do the portions of object 1602 that are farther away from return opening 1608. This is one of the factors that contribute to uneven heating of object 1602.
Thus, it is an object of the present invention to eliminate or reduce some of the inefficiencies when heating an object using a fluid.
It is yet another object of the present invention to optimize the efficiency of heating an object when using hot air impingement.
It is yet another object of the present invention to resolve the spotting problem without compromising the efficiency of heating an object using hot air impingement.
It is an even further object of the present invention to cause the air to travel along a uniform path from the time it enters the chamber until it reaches the return opening.
It is yet another object of the present invention to cause all portions of the surface of the object to be in contact with substantially the same volume of air.
It is yet another object of the present invention to reduce the pressure drop of the air from the time it enters the chamber until it reaches the return opening.
Other objects and advantages of the present invention will become apparent from the following description.